1. Field of the Invention
The present invention relates to an amplifier circuit and an input circuit, and particularly relates to an amplifier circuit and an input circuit capable of changing a gain.
2. Description of the Related Art
Apparatuses for processing video signals sometimes receive different levels of video signals depending on the type of devices connected thereto. The apparatuses therefore require input circuits for amplifying the input video signals to the level compatible with the devices and outputting the amplified signals.
FIG. 4 is a circuit diagram of a related-art input circuit 1.
The input circuit 1 comprises first and second amplifier circuits 11 and 12 with voltage gain different from each other, and switching circuits 13 and 14.
An input voltage Vin1 from an input terminal Tin1 is applied to the first and second amplifier circuits 11 and 12. The first amplifier circuit 11 amplifies the input voltage Vin1 applied from the input terminal Tin1 at a voltage gain Gv1. The output of the first amplifier circuit 11 is sent out from an output terminal Tout1 via the switching circuit 13.
The second amplifier circuit 12 amplifies the input voltage Vin1 applied from the input terminal Tin1 at a voltage gain Gv2. The output of the second amplifier circuit 12 is sent out from an output terminal Tout1 via the switching circuit 14.
The switching circuit 13 is switched on/off according to a gain control signal CNTL1. The switching circuit 14 is switched on/off according to a signal /CNTL1, which is an inverted signal of the gain control signal CNTL1. Therefore, the switching circuit 13 is ON when the switching circuit 14 is OFF, and the switching circuit 13 is OFF when the switching circuit 14 is ON.
When the switching circuit 13 is ON and the switching circuit 14 is OFF, the input voltage Vin1 of the input terminal Tin1 is amplified by the first amplifier circuit 11 with the voltage gain Gv1 and then output as an output voltage Vout1 from the output terminal Tout1. When the switching circuit 13 is OFF and the switching circuit 14 is ON, the input voltage Vin1 of the input terminal Tin1 is amplified by the second amplifier circuit 12 with the voltage gain Gv2 and then output as an output voltage Vout2 from the output terminal Tout1.
This input circuit 1 of FIG. 1 is not economical in power consumption, because both of the first and second amplifier circuits 11 and 12 are always driven.
FIG. 5 is a circuit diagram of another related-art input circuit 2. In FIG. 5, elements identical to those in FIG. 4 are denoted by the same reference numbers.
The input circuit 2 shown in FIG. 5 is provided with a switching circuit 21 disposed between the first amplifier circuit 11 and a supply voltage Vcc for providing a drive voltage to the first amplifier circuit 11 in place of the switching circuit 13, and a switching circuit 22 disposed between the second amplifier circuit 12 and a supply voltage Vcc for providing a drive voltage to the second amplifier circuit 12 in place of the switching circuit 14.
The switching circuit 21 is switched on/off according to a gain control signal CNTL2. The switching circuit 22 is switched on/off according to a signal /CNTL2, which is an inverted signal of the gain control signal CNTL2. Therefore, the switching circuit 21 is ON when the switching circuit 22 is OFF, and the switching circuit 21 is OFF when the switching circuit 22 is ON.
When the switching circuit 21 is ON and the switching circuit 22 is OFF, the first amplifier circuit 11 is in an operating state and the second amplifier circuit 12 is in a suspended state. The input voltage Vint1 input from the input terminal Tin1 is amplified by the first amplifier circuit 11 with the voltage gain Gv1, and then output as an output voltage Vout1 from the output terminal Tout1. On the other hand, when the switching circuit 21 is OFF and the switching circuit 22 is ON, the first amplifier circuit 11 is in the suspended state and the second amplifier circuit 12 is in the operating state. The input voltage Vint1 input from the input terminal Tin1 is amplified by the second amplifier circuit 12 with the voltage gain Gv2, and then output as an output voltage Vout2 from the output terminal Tout1.
The input circuit 2 can reduce the power consumption, because it can suspend the operation of either one of the amplifier circuits 11 and 12 by having the switching circuit 21 disposed between the supply voltage Vcc and the first amplifier circuit 11 and the switching circuit 22 disposed between the supply voltage Vcc and the second amplifier circuit 12.
However, this type of amplifier circuit needs to have plural differential amplifier circuits each having a resistance to set a specified gain, and therefore increases the size of the circuit as well as, if applied to IC chips, the chip area.
Also, a complicated control mechanism is required in order to operate one differential amplifier circuit and to stop the rest of the differential amplifier circuits by suspending the power supply thereto.